Membrane filtration processes provide attractive solutions for various water treatment applications such as used for the removal of inorganic constituents and in particular in desalting brackish water and seawater and for the removal of Synthetic Organic Chemicals (SOC). Since traditional water treatment methods are not always able to meet the requirements imposed by the drinking water regulations, membrane filtration processes are becoming preferable in such applications, particularly due to their small space requirement and efficient removal of contaminants.
Pressure-driven membrane processes are defined as processes in which the feed stream is fed at a volumetric rate Qf into a membrane device (e.g., pressure vessel) equipped with membranes that divide the device space into a feed side and a permeate side, and in which a pressure difference across the membranes causes the solvent (usually water) to pass from the feed space to the permeate space at a volumetric rate denoted as Qp. The remaining solution which is now concentrated in the rejected solutes, leaves the feed space of the membrane device as a concentrate stream at a volumetric rate denoted as Qr. The fraction of feed that leaves the membrane device as permeate is referred to as the membrane recovery, Y:
                    Y        ≡                              Q            p                                Q            f                                              (        1        )            
As recovery is increased the concentration of rejected solutes in the concentrate stream, Cr, is given by mass balance as:
                              C          r                =                              C            f                    ⁢                                    1              -                              Y                ⁡                                  (                                      1                    -                    R                                    )                                                                    1              -              Y                                                          (        2        )            
Pressure-driven membrane processes are distinguished from each other by the types of solutes they reject or pass to the permeate side. For a given process this selectivity can be defined by the empirical solute rejection for each solute i, Ri, defined by:
                                          R            i                    ≡                                                    C                                  f                  ,                  i                                            -                              C                                  p                  ,                  i                                                                    C                              f                ,                i                                                    =                  1          -                                    C                              p                ,                i                                                    C                              f                ,                i                                                                        (        3        )            
Where Cfi, Cpi refer to the feed and permeate concentrations respectively of component i. For highly rejected species, R≅1 and then the relation defined in equation 2, between its concentration in the concentrate stream, Cr, and the recovery, Y reduces to:
                                          C            r                    =                                    C              f                        ⁢                          1                              1                -                Y                                                    ,                              where            ⁢                                                  ⁢            R                    =          1                                    (        4        )            
Typical definitions of the various types of pressure driven processes are provided in table 1:
TABLE 1Definition of various pressure driven membrane processes,adapted from M. Mulder, Basic Membrane Technology,2nd ed., Ch. 1 (Marcel Dekker, 1996).Water permeabilityrangeMembrane processSolutes rejected(L/m2-h-bar)microfiltration (MF)dp > 0.1-1 μm50<ultrafiltration (UF)MWCO > 1000-250,00010<nanofiltration (NF)multivalent ions, MWCO >2-20200-1000reverse osmosis (RO)monovalent ions0.05-7   
In this table dp refers to the diameter of the solute rejected. MWCO refers to the molecular weight cutoff usually defined as the molecular weight of the solute which is 90% or more rejected by the membrane. It can be seen that the pressure-driven membrane processes that will result in partial or complete desalination are reverse osmosis (RO) and nanofiltration (NF). They will also remove natural organic matter, synthetic organic matter, and inorganic chemicals, and thus they are suitable for partial or complete desalting of brackish and sea water.
The rate at which solvent together with non-rejected solutes passes through the membrane per unit membrane area is defined as the membrane flux (denoted Jv) with units of volume/(unit area-time). During the filtration process the membranes become fouled and as a result less effective. Membrane fouling has become one of the primary impediments to their acceptance in water treatment applications. The occurrence of membrane fouling in reverse osmosis and nanofiltration processes leads to reduction in the production rate and sometimes to loss of solute rejection. There are several kinds of fouling which typically occur in such processes:                colloidal fouling;        organic fouling (adsorption of soluble organics on membrane surface);        biofouling—formation of a biofilm which by itself or in concert with other kinds of fouling causes deterioration of membrane performance; and        precipitation fouling (or scaling) due to precipitation of sparingly soluble salts and minerals.        
Common methods for preventing precipitation fouling are to limit the system recovery, Y, by keeping permeate rate below a certain maximum fraction of the feed rate, such that the concentrations of sparingly soluble salts do not greatly exceed saturation in the concentrate end. These saturation limits can be moderately increased by feeding antiscalants to the membrane feed end which increase chemical pretreatment costs. In addition, a requirement imposed by the membrane manufacturers requires that a minimum tangential flow rate exist in each commercial spiral element in order to minimize concentration polarization caused by the buildup of rejected salt convected to the membrane surface by the flux (e.g. for 8 inch diameter spiral NF or RO elements, some manufacturers recommend a minimum flow rate of 45 L/min).
However, in the new low-pressure LPRO (Low-Pressure Reverse Osmosis) and LPNF (Low-Pressure Nanofiltration) membranes, that operate at pressures of 3-10 bar, the axial pressure drops along the feed paths of the membrane elements can significantly reduce the driving force for product water permeation, which requires that flow rates will not be too high through the membrane elements. These conflicting requirements make design of new water treatment plants quite difficult, which resulted in various new strategies that have been proposed (for conventional ways of dealing with this see e.g., “Innovative System Designs to Optimize Performance of Ultra-low Pressure Reverse Osmosis Membranes”, Nemeth, J., Desalination, 118, 63-71, 1998).
Other ways to control membrane fouling utilizes hydrodynamic and chemical methods, periodic backwashing, chemical cleaning, changing operating conditions, and reducing the operating flux. Another solution for controlling membrane fouling proposes changing the flow direction in order to reduce concentration polarization and fouling in general (“Ultrafiltration Membranes and Applications”, Breslau, B. R. at al, Polymer Science and Technology, Plenum Press, Vol. 13; “Flux Enhancement Using Flow Reversal in Ultrafiltration”, Hargrove, S C and Ilias, S., Sep. Sci. Technol., 34 (6&7), 1319). However none of these publications teach or propose a solution for preventing precipitation fouling.
A flow reversal process and device are described in U.S. Pat. No. 5,690,829 (to Lauer), which particularly relates to the cleaning of the membrane from dirt particles. Another possible solution for reducing membrane fouling is described in U.S. Pat. No. 5,888,401 (to Nguyen), which suggests periodically increasing the permeate pressure next to the membrane by partially closing a valve on the permeate side, which results in reductions in the permeate flow rate. This last method reduces the rate of overall permeate recovery which is a disadvantage.
As will be apparent to those skilled in the art, an efficient solution for preventing precipitation fouling has also implications for biofouling since the stagnant layers and surfaces of scale layers can allow biofilms to attach and develop with less shear forces to remove them.
In many processes sparingly soluble salts can limit the recovery of desalination processes as their concentration increases in the brine as more product water is pulled out of the feed flow. Different techniques have been used to cope with this problem (Section 9.4 in Water Treatment Membrane Processes, Mallevialle, J., Odendaal, P., Wiesner, M. eds., McGraw-Hill, 1996). Chemical softening has been proposed to precipitate sparingly soluble salts most of which are salts of alkali earth metals (Ca, Sr, Mg, Ba). The problem with this approach is that it requires stoichiometric amounts of chemicals to precipitate all of the metal ions of sparingly soluble salts, which is often costly. For example, brackish water containing 100 mg/L of calcium, 30 mg/L of magnesium, and 150 mg/L of carbonate alkalinity as bicarbonate will require 91 g/m3 of hydrated lime and 135 g/m3 of soda ash to completely remove the calcium. At 80 $/ton for hydrated lime and 180 $/ton for soda ash this would involve a chemical cost of 1.8 cents/m3. In addition the sludges formed in lime softening are often voluminous and hard to remove. This can be prevented by using advanced precipitation processes that combine precipitation softening with microfiltration, also known as Membrane Assisted Crystallization (MAC) or filtering through a filter cake of calcium carbonate seeds which is also known as Compact Accelerated Precipitation Softening (CAPS).
Alternatively the pH can be reduced by adding acid and removing the carbonic acid formed by air stripping. This type of treatment eliminates the carbonate scales problems but the problems associated with sulfate scales will still remain. In the previously mentioned example of brackish water, it would be necessary to add 120 g of sulfuric acid//m3 of feed water to completely remove carbonate alkalinity. Meanwhile this would raise the risk of exceeding calcium sulfate solubilties.
Consequently, the most common approach today is to use antiscalants which allow operation at various values of super-saturation. However even with the most advance antiscalants used today, there are limits on the super-saturation ratios (for example ˜2.60-3.0 for calcium sulfate, a Langlier Saturation Index (LSI—log10 of supersaturation ratio) of 2.8 for calcium carbonate, and supersaturation ratio of 2.0 for silica. This often means that recoveries are often limited to 75-90%.
Alternatively, a NF process may be used to remove the hardness ions, and in this case the permeate from the nanofilter can then be fed to RO or thermal desalination units to recover the desalinated water at fairly high recoveries. However because counterions of the alkali earth metals are also rejected in this process, super-saturation conditions are also reached in the NF process if the recoveries are high enough.
It is important to reach high recoveries in the NF process since the overall recovery in two stages in series will be the product of the two steps. For example, if the recovery of the desalination process is 95% and the recovery of the NF process is 90%, then the overall recovery will be 85.5%. There is no considerable improvement in this result in comparison with the recoveries obtained in standard RO processes. One way of overcoming this problem is to recycle the concentrate of the desalination step to the nanofilter. However, by doing so the average salinity in the desalination process is raised with attendant increase of salinity in the product. A number of researches have proposed using precipitation softening on the concentrate before conducting further desalination on the treated concentrate (Enhanced Water Recovery from Primary Membrane Desalination Concentrate by Precipitative Softening and Secondary Membrane Desalination, Rahardianto, A., Cohen, Y., and Williams, M. D., paper 394e, AIChE Fall meeting, 2004.) in order to increase the recovery. However such a treatment is complicated by the presence of antiscalants in the concentrate.
In view of the aforementioned problems there is a need for filtration processes capable of effectively and efficiently controlling and preventing precipitation fouling in pressure-driven membrane desalination processes, and of improving the recoveries of such processes.
It is an object of the present invention to provide a high-flux filtration process for controlling and preventing precipitation fouling in pressure-driven membrane processes.
It is another object of the present invention to provide a method and system for efficiently controlling flow reversal in a membrane filtration process for preventing precipitation fouling in pressure-driven membrane processes.
It is still another object of the present invention to provide a method and system for preventing precipitation fouling in a membrane filtration process wherein the composition of the solution next to the active membrane surface exceeds the effective saturation limit of sparingly soluble salts.
An additional object of the invention is to provide a method and system for achieving high recoveries in a membrane filtration process operating with high local super-saturations.
Other objects and advantages of the invention will become apparent as the description proceeds.